This is invention relates to electroluminescent (EL) lamps and, in particular, to an EL lamp having an integral fuse and connector.
An electroluminescent lamp is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is transparent. The dielectric layer may include a phosphor powder or there may be a separate layer of phosphor powder adjacent the dielectric layer. The term "electroluminescent dielectric layer" is intended to be generic to either construction. The phosphor powder radiates light in the presence of a strong electric field, using very little current. The front electrode is typically a thin, transparent layer of indium tin oxide or indium oxide and the rear electrode is typically a polymer binder, e.g. polyvinylidene fluoride (PVDF), polyester, vinyl, or epoxy, containing conductive particles such as silver or carbon. The front electrode is applied to a polymer substrate, such as polyester or polycarbonate, which provides mechanical integrity and support for the other layers.
In order to produce a sufficiently strong electric field within the phosphor to cause the phosphor to glow brightly, an EL lamp is typically connected to a source of alternating current of 80 volts or more. Although an EL lamp draws very little current, typically one milliampere per square inch of light emitting surface, an EL lamp is often connected to a source of power capable of supplying hundreds or thousands of times as much current, e.g. an AC outlet in a home. It is therefore a problem if an EL lamp becomes damaged because a large current is available for any low resistance path which may be created, e.g., by accidentally puncturing or cutting an EL lamp. A low resistance path may be created that is not sufficient to open a fuse rated at fifteen or twenty amperes but is sufficient to produce a considerable amount of heat.
It is known in the art to combine a fuse with an EL lamp. U.S. Pat. No. 3,109,959 (Delachapelle et al.) discloses an EL lamp having a metal clip connector in which a portion of the clip is narrowed to form a fuse. The fuse is disclosed as having a rating of ten amperes.
Providing a fuse for an EL lamp can significantly affect the cost of the lamp. High amperage fuses, e.g. fuses rated at three amperes or more, are relatively inexpensive. Low amperage fuses are more difficult to make and are more expensive than high amperage fuses. A fuse rated at a few milliamperes or a few tens of milliamperes would cost much more than the EL lamp it was protecting.
A low amperage fuse is disclosed in U.S. Pat. No. 4,208,645 (Harmon et al.) in which conductive ink is screen printed on an oriented plastic substrate. The fuse has two failure modes. For low overcurrent, the substrate melts, separating the conductive trace printed on the substrate. For high overcurrent, a portion of the conductive trace evaporates from the substrate. Although lower in cost than high current fuses, the fuse described in the Harmon et al. patent, like the fuse described in the Delachapelle et al. patent, is a separate component which must be added to the device to be protected.
One type of connector used in the prior art is a staple type of connector having several small blades punched from a conductive sleeve. The blades puncture the lamp and are bent over to secure the connector to the lamp. A connector must be secured to a lamp without destroying the conductive stripe to which contact is made.
Another type of connector used in the prior art is a combination of a pin and an eyelet or grommet. A hole is punched in the lamp and the eyelet is inserted through the hole and crimped into place. The pin is then press fit into the eyelet, completing the connector. This type of connector is more expensive than the sleeve described above.
In many applications for EL lamps, such as the dial lamp in a household clock, cost is a major consideration. The electrical connection to a clock motor is typically made with a ferrule or collar crimped about the wire from a line cord and a terminal within the clock. Some EL lamps used as a dial lamp in clocks include a sleeve type of connection that is crimped to a terminal with a wire from a line cord. Although part of the sleeve is crimped, the puncture of the blades remains the mechanism by which the EL lamp is connected to a source of power and this mechanism is not particularly reliable.
In view of the foregoing, it is therefore an object of the invention to provide an integral fuse for an EL lamp.
Another object of the invention is to provide a fuse which does not increase the cost of the EL lamp to which it is attached.
A further object of the invention is to provide an integral connector for an EL lamp.
Another object of the invention is to provide a connector which does not increase the cost of the EL lamp to which it is attached.
A further object of the invention is to provide a low amperage fuse for an EL lamp.